Conventionally, there has been disclosed an illumination apparatus which includes a constant current circuit employed for supplying a constant current to an LED (light emitting diode); and a phase angle control circuit for adjusting a firing angle of a triac wherein the LED, the constant current circuit and the triac are connected in series (see, e.g., Japanese Patent Application Publication No. 2009-200257 (JP2009-200257A), FIG. 1 and claim 1).
The illumination apparatus disclosed in JP2009-200257A is capable of dimming control and can prevent a current larger than a predetermined level from flowing through the LED even when a power source voltage is varied, for example. Conventionally, there has been employed a dimming operation of an incandescent bulb performed by the triac. The dimming control can be realized merely by replacing an existing wall-installed switch with a dimmer having the triac. The incandescent bulb, however, tends to be replaced recently with an LED lamp, which may cause a malfunction in the dimmer having the triac.
When the dimmer having the triac malfunctions, it may be difficult to maintain an ON state of the triac and, resultantly, the triac may become turned off. This is because ah output current from the triac drops below a holding current needed for maintaining the ON state of the triac.
The reasons for the malfunctions of the triac may be described as follows. First, since a power consumed by an LED lamp is small, the output current from the triac can become easily smaller than the holding current of the triac. Second, due to a circuit configuration of the LED lamp, an input current becomes discontinuous and reduced to become smaller than the holding current of the triac, whereby the triac is turned off.
For these reasons, a current dividing circuit including a resistor may need to be provided in the LED lamp in order to increase the current flowing through the LED lamp to be greater than the holding current of the triac, or a power factor improving circuit or a dither rectifying circuit needs to be provided in order to prevent the input current from being discontinuous. Alternatively, the operation of the triac may be stabilized by providing a resistor or an incandescent bulb which is connected to the load in parallel.
In brief, it is possible to prevent the triac from malfunctioning by designing the load current to be greater than the holding current of the triac. However, when a current dividing circuit including a resistor is provided in the LED lamp to increase the LED lamp current to be greater than the current of the triac, stresses are generated in circuit elements due to heat produced from the resistor of the current dividing circuit.
In the meantime, to meet a demand for decrease in size and manufacturing cost of the LED lamp, attempts have been made to simplify a heat dissipation structure or a circuit. However, if heat is generated in the current dividing circuit for preventing the malfunction of the triac, it becomes difficult to reduce the size and the cost of the LED lamp.
Further, when the power factor improving circuit is provided in the LED lamp, the cost of the LED lamp increases and it becomes difficult to scale down the LED lamp. In addition, when a resistor is connected to the load in parallel, the LED lamp may malfunction due to fluctuation of the input voltage. This results when a voltage from a commercial power source is low, causing a current flowing through the resistor to be reduced. For that reason, the resistor may be designed to have a small resistance. This, however, increases power loss when a higher power source voltage is supplied.